The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

EFFECT OF EXPANSION. 281
We have stated that a considerable increase of power,
from a given quantity of steam, was produced by cutting off
the steam after the piston had made a part of its descent,
and allowing the remainder of the descent to be produced
by the expansive force of the steam already admitted. We
shall now more fullly explain the principle on which this
increase of power depends.
Let A B, (fig. 69,) as before, represent a tube, the bottom
of which is equal to a square foot, and let P be a piston in it,
resting upon a cubic inch of water spread over the bottom;
and let w be an empty vessel, the weight of which exactly
counterpoises the piston. By the application of the lamp,
the water will be converted into steam of the atmospheric
pressure, and the piston will be raised from P to P', through
the height of one foot, the space in the tube beneath it being
filled with steam, and the vessel w will have descended
through one foot. Let half a ton of water be now poured
into the vessel w; its weight will draw the piston P' upward, so that the steam below it will expand into a largei
space. When the piston P' was only balanced by the empty
vessel w, it was pressed downward by the whole wveight
of the atmosphere above, which amounts to about one ton:
now, however, half of this pressure is balanced by the half
ton of water poured into the vessel w; consequently the
effective downward pressure on the piston P' will be only
half a ton, or half its former amount. The piston will there-,
fore rise, until the pressure of the steam below it is diminished
to the same extent. By what has been already explained,
this will take place when the steam is allowed to expand
into double its former bulk; consequently, when the piston
has risen to P", one foot higher, or two feet from the bottom
of the tube, the steam will then exactly balance the downward pressure on the piston, and the latter will remain sta
tionary; the vessel w, with the half ton of water it contains,
will have descended one foot lower, or two feet below its first
position. Let the steam now be cooled and reconverted into
2A2 36